msm: mdss: Add debug suppport for FRC

Add debug facility for deterministic frame rate control (FRC),
FRC is to maintain specific video cadence based on its frame
rate and display refresh rate.

This change is to collect statistics w.r.t video FRC such as
when video frame is kicked off and when vsync is triggered. Use
these info to check whether FRC is working as expected.

Usage:
  - start logging: echo 1 > /d/mdp/frc/log
  - stop logging:  echo 0 > /d/mdp/frc/log
  - dump logs:     echo 1 > /d/mdp/frc/dump
  - disable FRC:   echo 0 > /d/mdp/frc/enable

Change-Id: Icc2f487681d0efb760999b30724c8ad4256e8130
Signed-off-by: Ray Zhang <rayz@codeaurora.org>